Spectral modification through phase modulation with spatial extent

ABSTRACT

An optical pulse propogating in an optical guide of an electro-optic medium interacts with a coplanar RF traveling wave to shift the optical wavelength. The RF phase is resynchronized to the optical pulse phase with spatial periodicity to provide a desired phase relationship between RF and optical radiation. A unidirectional incremental wavelength shifter comprises a frequency multiplexer for soliton communication and a symmetrical sideband modulator of this type varies the spectral width of probe illumination in an atomic absorption spectrometer probe beam.

FIELD OF THE INVENTION

The invention relates to the field of optical spectrum altering deviceswith special applications to atomic absorption spectroscopy and tosoliton based communication.

BACKGROUND OF THE INVENTION

It is known in the prior art to propagate an optical signal through anelectro-optic crystal where a radio frequency, RF field is imposed onthe crystal. The RF phase is adjusted to modulate the refractive indexof the electrooptic medium to adjust the phase approximately linearlywith time for the propagating optical pulse. The decreasing optical pathproduces an effect similar to that of uniform motion of the opticalsource toward the observer, e.g., a doppler upshift. A 180° shift in theRF phase creates the effect of uniform motion away from the observer(doppler downshift). See Duguay and Hansen, IEEE Journal of QuantumElectronics, QE-4, p. 477, (1968). The effect is reviewed by Kaminow andTurner, Proc. IEEE, vol. 54, pp. 1122-1124 (1966).

In the present work, we describe a different structure wherein an RFtraveling wave in an RF guide structure co-propagates with an opticalpulse in an optical guide incorporating an electro-optic medium. The RFphase is adjusted in relation to the co-propagating pulse to produce adesired refractive gradient in the electro-optic medium whereby theoptical pulse experiences a uni-directional wavelength shift of desiredmagnitude. In contrast to the lumped element arrangement of prior art(for which the refractive index is constant in space) the presentinvention yields a refractive index which is a function of space andtime.

A very useful arrangement integrates, on a single substrate, theelectro-optical modulator with the RF transmission line for use as acomponent in particular applications.

Consider a light pulse propagating through a nonlinear medium whichexhibits a linear electro-optic coupling. A microwave signal E_(m)propagates coincidentally with the optical pulse. The microwave field

    E.sub.m =E.sub.0 sin (kz-ωt)                         Equ. 1

produces a dynamic effect upon the refractive index of the nonlinearmedium given by

    n=n.sub.o +δn

    δn=-(n.sub.0.sup.3 r.sub.c E.sub.m)/2                Equ. 2

where n₀ is the unshifted refractive index of the medium and r_(c) isthe effective first order electro-optic coefficient. Assume that thelight pulse is spatially coincident with the microwave phase asillustrated in FIG. 1. As shown, the pulse is so located in time andspace that it coincides with an increasing slope of the field E_(m) andthus a reduced index of refraction at its trailing edge and an increasedindex of refraction at its leading edge, both instantaneously andlocally. This results in the leading edge of the pulse traveling at areduced velocity with respect to the trailing edge. As a result, thepulse is spatially squeezed. Although the pulse is spatially squeezed,the total number of wave periods or cycles remains the same, yielding afrequency upshift. A frequency downshift may be obtained bysynchronizing the optical pulse with the opposite RF phase, e.g., bylocating the optical pulse in a region of decreasing electric fieldE_(m). The leading edge of the pulse then propagates at a relativelygreater velocity than the trailing edge because the leading edgeexperiences a lesser index of refraction. Thus the pulse is spatiallystretched, the same number of optical cycles occupying a greater spatiallength.

In this discussion, it is sufficient to assume a positiveelectro-optical coefficient although that nothing herein is intended tolimit the nature of the medium or the operating range wherein thecoefficient has a specific sense.

In general, the refractive index is a function of the wavelength,especially when comparing optical and microwave radiation. As aconsequence the optical propagation velocity (more specifically, thegroup velocity) is usually different from the phase velocity of the RFwavefront. In one embodiment of the present invention, this differencemay be ameliorated by introducing period polarity reversals in thetransmission line.

The present work embraces both a uni-directional incremental wavelengthshift and bidirectional shifting similar to symmetrical sidebandmodulation. The uni-directional embodiment of the wavelength shifter ofthe present invention is useful in a wide range of measurementapplications. The symmetrical bidirectional embodiment is also wellsuited for instrumentation as described herein.

In the prior art there has been an application for controlled wavelengthshifting in the optical region in the area of atomic absorptionspectrometry. It is known in that work to employ the Zeeman (or Stark)effect to shift the emission wavelength of a probe beam obtained from ahollow cathode lamp or the absorption wavelength of a test sample underthe influence of an external field as described by U.S. Pat. No.4,341,470. An electro-optic wavelength shifter has been described for asimilar application by Cammann (U.S. Pat. No. 4,834,535). In the latterwork an optical modulator is subject to an applied RF field and theoptical beam is caused to repeatedly traverse the modulator. The RFfield of the reference necessarily has a wavelength which is largecompared to the dimensions of the modulator in contrast to therequirements of the present invention wherein an RF traveling wavelocally modifies the optical properties of the modulator to provide aspatial and temporal modulation of the optical properties of theelectro-optical medium.

It is known to employ the propogation of optical solitons on an opticfiber to support a communication system exhibiting unusually high bitrates. Such a system was studied and described by Mollenauer, Gordan andIslam, IEEE J. of Quant. Elect., vol. QE-22, pp. 157-173 (1986). Apractical system of this type requires multiplexing of differentchannels on the same physical fiber. A frequency multiplexing anddemultiplexing apparatus utilizing the present invention is describedbelow.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relative alignment of optical pulse and RF wavefrontfor an upshift of optical frequency.

FIG. 2a shows a preferred 1/2 CPW integrated RF-optical guide of thepresent invention.

FIG. 2b shows another arrangement for introducing the optical phaseshift in the present invention.

FIG. 2c shows an alternative for introducing phase shifts in thetraveling RF wavefront.

FIG. 2d shows a single ended geometry for a wavelength shifter of thepresent invention.

FIG. 3a illustrates apparatus employed to demonstrate the invention.

FIG. 3b is a double exposed photograph demonstrating a frequency upshiftwith the apparatus of FIG. 3a.

FIG. 3c is a double exposed photograph demonstrating a frequencydownshift with the apparatus of FIG. 3a.

FIG. 4 shows measured dependance of the frequency shift on microwavepower for the apparatus of FIG. 3a.

FIG. 5 is a block diagram of an atomic absorption spectrometerincorporating the present invention.

FIG. 6 is a block diagram of a soliton based communication systemincorporating the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIG. 1 there is shown a representation of the desiredphase relationship to be maintained between an optical pulse (orwavefront) 12 and traveling microwave wavefront 14. As shown, theleading edge and trailing edge of the optical pulse concurrentlyexperience a differential optical density which slows the leading edgerelative to the trailing edge because the relative magnitude of therespective local electric fields so effects the electro-optic medium toreduce the refractive index locally in proportion to the local electricfield. In the relationship shown the result is to compress the pulseproducing an upshift in the optical frequency. If the relative phasewere to be maintained with the optical pulse relatively phased as shownby pulse 16, the result would be to produce a frequency downshift.

The difference in phase velocity between the propagating RF wavefrontand the group velocity of the optical pulse propagating in the opticalguide may be treated, especially in the case of a non-TEM RF waveguide,by selection of operating point in the neighborhood of the cutoff pointwherein the phase velocity varies over a range including the desired RFphase velocity. With selected design conditions, an arrangement may befound wherein the drift in phase difference between the RF wave andoptical pulse amplitude envelope propagation may be held to a desiredminimum.

Because of the difference in phase velocity of the propagating RFwavefront and the group velocity of the optical pulse, the phaserelation of FIG. 1 will be lost as a function of distance along thepropagation axis. In a preferred embodiment of the invention the phaserelationship is periodically restored by the expedient of reversing thesense of the RF electric field with appropriate spatial periodicity. Thedistance L forming a spatial period between polarity reversals of thistype to obtain a 180° phase shift is given by ##EQU1## where f is themicrowave frequency and n is the optical refractive index, ε_(eff) isthe effective dielectricconstant describing describing the phasevelocity of the RF wave. Here, the phase velocity of the RF wave isgiven by c/√ε_(eff).

FIG. 2a shows the arrangement by which a preferred embodiment of the RFoptical guide has been achieved with integration on a lithium niobatesubstrate 20. A half CPW geometry comprises RF propagation path 22displaced from ground plane 24. Optical path 26 is defined in thesubstrate 20 with a titanium diffusion of 6μ width. The RF path 22 andground plane 24 are segmented and geometrically alternate in symmetryabout optical guide 26. Lithium niobate exhibits a refractive index of2.25 for 1.06 micron wavelength z polarized radiation propagating alongthe y axis. For microwave radiation, the dielectric constant is about35. Thus the phase velocity of the microwave signal is slower than boththe phase and group velocity of the optical signal by about a factor of2a. The disparity in velocity of the optical and microwave signals isremedied by implementing periodic polarity reversals as shown in FIG.2a. An oxide insulating layer 28 is disposed over the optical guide 26and substrate 20 and the RF conductor 22 is thereby isolated from theoptical guide 26 allowing the crossover portion 30 invert the conductor22 about the propagation axis. An airbridge 32 serves the concomitantpurpose for the ground plane 24. For one experimental device the spatialperiod L was chosen to be 4.34 mm. A reversal gap 33 between sectionsoccupied 60 microns along the axis.

Alternate arrangements for re-synchronizing the RF and optical phases inthe combined RF/optical transmission line are discussed below.

At FIG. 2b band section AA thereof there is shown an RF conduction path62 which exhibits delay segments 64 designed to introduce a sufficientdelay to realize a phase shift of substantially 180° in respect of theoptical pulse propagating on optical guide 66. Optical guide 66 isdisposed between RF path 62 and ground plane 68.

Yet another alternate arrangement is illustrated in FIG. 2c where a halfCPW transmission line includes lumped capacitances 70 implemented inknown manner to achieve a phase shift of 180° for the propagating RFwavefront. One skilled in the art will recognize that a number ofapproaches may be taken to realize the desired phase shift forre-synchronizing the RF energy and optical pulses. Both of thesealternate arrangements require significant additional processing toconstruct the integrated device.

The geometry of the wavelength shifter of the present invention may takethe form of the single ended embodiment shown in FIG. 2d. The wavelengthshifter 92 as previously described, receives incident light from source90 and both optical and RF radiation are reflected by symbolicreflecting means 94 back through the shifter 92. A frequency selectiveelement 96 selectively reflects (as in the example FIG. 2d) opticaloutput of frequency greater than a selected value. If the selectedcritical frequency for element 96 corresponds to a value slightlygreater than c/τ_(c), where c is the coherence length and τ_(c) is theline width of the optical transition, then the shifted wavelength willbe available over a substantial range. Notice that passage through adoubled path length will produce twice the frequency shift asexperienced over the single transit geometry.

The preferred embodiment of FIG. 2a has been tested to demonstratewavelength shifting capabilities. The test apparatus is shown in FIG. 3awhere a Nd:YLF laser 36 generates 60-ps optical pulses of 1.054 nmwavelength at 75.7 MHz repetition rate which are compressed to 10 pspulse width by pulse compressor 37. RF synthesizer 38 establishes thelaser pulse repetition rate and correlates the phase of the microwavesynthesizer 42. The microwave output of synthesizer 42 is amplified inTWT amplifier 43 and then applied to the microwave transmission line ofthe waveshifter 41. The compressed optical pulses are directed throughoptical polarizing fiber 39 to the wavelength shifter 41 and thence tooptical spectrum analyzer 45, the output of which is displayed onoscilloscope 46. The wavelength shifting thus achieved is reproduced atFIGS. 3b and 3c. These figures are double exposed photographic recordsof the optical spectrum acquired from an optical spectrum analyzer. Thedouble exposure superimposes the the unshifted optical spectrum centeredin the image and the shifted optical spectrum obtained by applying RFpower to the device. The only variable adjusted between the tests ofFIGS. 3b and 3c is the phase of the signal derived from microwavesynthesizer 42. The microwave power was approximately 16 W at 17.2596MHz, which corresponds to the 228^(th) harmonic of the laser repetitionrate. Each horizontal division of the oscilloscope display of FIGS. 3band 3c represents 265 GHz.

For small wavelength shifts the change in frequency should be linear inthe microwave electric field or should scale with the square root of themicrowave power as shown by equation 1 and 2. FIG. 4 demonstrates thescaling of the frequency shift with microwave power in agreement withtheoretical predictions

FIG. 5 shows a schematic arrangement for an atomic absorptionspectrometer incorporating a wavelength shifter as herein described. Anincoherent light source 82 is directed to propagate through polorizer 98and then into an optical guide 84 comprising an electro-optic medium,such as by way of example, lithium niobate. The optical guide isdisposed together with an RF transmission line or guide 86 such that anRF traveling wave propagating on transmission line 86 influences theelectro-optic medium to locally modify the optical properties thereof.The RF source 88 includes means to tune the RF energy in frequency andphase to obtain desired values. The optical output of the modulator isthen directed through collimator lens 90 to transit an atomized samplein atomizing apparatus 92. In one arrangement this atomizing apparatusincludes a carbon rod furnace and associated equipment for treating asample to produce an atomic vapor. A focusing lens 94 images the lightsource on monchromator 96. The analyzed optical signal is then convertedto an electrical signal by detector 100.

In the context of this AA application either a cw mode of operation or apulsed mode may be used, but a cw embodiment is usually preferred. Thecw optical energy intensity is phase modulated with the RF travelingwave in the wavelength shifter of the invention to produce a temporallyvarying optical wavelength. The spectral content of the optical beamderived from the wavelength shifter will consist of a pair of sidebandsλ₀ ±λ. By modulating the phase shift it is possible to concentrate theoptical power into sidebands of the optical carrier under the conditionthat the Bessel function representing the modulation of the carrier atmodulation index m, i.e., J₀ (m) vanishes. The unshifted wavelength willbe effectively suppressed if the microwave frequency f>>1/τ where τ isthe homogeneous linewidth for the transition. The microwave period isthus short in comparison with the optical transition lifetime. Theoptical spectrum is distorted to localize the spectral density into twolobes symmetrically displaced with respect to the center frequency. Theposition of these lobes is thus variably displaceable as the wavelengthis (symmetrically) shifted to sweep over the spectral region of interestin the AA measurement. This mode of operation is analogous to the effectachieved in the prior art U.S. Pat. No. 4,341,470 utilizing thelongitudinal normal Zeeman effect of the light source although thephysical phenomenum is quite distinct. In the present embodiment themodulation of phase in the phase shifter of the present invention isseveral orders of magnitude greater than could be achieved using themagnetic field switching to carry out the analogous Zeeman effecttype-measurement.

Another particularly useful application for the wavelength shifter heredescribed occurs in the context of a multi-channel optical communicationsystem utilizing a soliton optical source. Such systems are capable ofrepeaterless operation over thousands of miles and thus represent aconsiderable economy for terrestrial communication systems. FIG. 6describes this context schematically for a frequency division multiplexarrangement. Soliton source 120 provides pulsed energy which is dividedinto a number of subchannels by known fanout means 102, in each of whichchannels the respective soliton pulse traverses the corrrespondingwavelength shifter 104_(i) herein described. The wave shifted soliton isthen directed to the channel information modulator 106_(i). Therespective subchannels are then recombined in known manner at combiner107 for transmission over optical fiber 108. Demultiplexing isaccomplished by analyzing the combined optical signal derived from theoptical fiber 108 as for example by means of diffraction grating 110.The wavelength resolved optical energy is collected by a correspondingdetector 112_(i) for further processing within that subchannelcommunication destination. In each subchannel the presence or absence ofa soliton pulse at the respective clocked period for the correspondingsubchannel encodes binary information for that subchannel as presentlypracticed in analogous communication systems.

Other soliton communication arrangements based upon the wavelengthshifter of the present invention include modulation of the individualsoliton to impress a proportional digital signal thereon. For example ina simple arrangement, a ternary code is supported by the three possiblestates of upshift, downshift or unshifted frequency of the soliton.Other schemes of encoding information will readily occur to the workerskilled in the art.

In a soliton communication system, the optical medium will support anumber of channels for which there may occur superpositions of solitons.Bidirectional communication traffic assures that such superpositions mayoccur at high rates. The frequency displacement obtained with thepresent invention for colliding pulses minimizes any substantialinteraction therebetween with a minimal incidence of crosstalk.

While there have been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and and describedmay be made without departing from the true spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. An optical wavelength shifting apparatuscomprising:(a) an optical guide means comprising an electro-optic mediumto support an optical pulse propogating therethrough, (b) a travelingwave guide structure for propogating an RF wave, said optical guidedisposed in relation to said traveling wave guide structure whereby theelectro-optic medium locally responds in proportion to the correspondinglocal electric field set up by the traveling wave to shift thewavelength of said traveling wave, said optical pulse comprising awavefront synchronized with said RF traveling wave whereby the leadingedge of said wavefront and the trailing edge of said pulse aresubstantially within the same RF quadrant, said quadrant occupying thephase region between extrema of said RF wave.
 2. The wavelength shiftingmeans of claim 1 wherein said traveling wave guide structure comprises atranmission line.
 3. The wavelength shifting apparatus of claim 2further comprising phase synchronization maintenance means forsubstantially maintaining said desired phase relationship.
 4. Thewavelength shifting means of claim 3 wherein said maintenance meanscauses a spatialy periodic polarity reversal of the electric field ofsaid traveling wave.
 5. The wavelength shifter of claim 4 wherein saidspatial period is equal to the length over which said optical pulse andRF traveling wave drift in relative phase by substantially 180°.
 6. Thewavelength shifter of claim 1 wherein said desired quadrant includes theregion of phase wherein the electric field of said RF wave isincreasing, whereby said optical pulse is upshifted to a higher opticalfrequency.
 7. The wavelength shifter of claim 1 wherein said desiredquadrant includes the region of phase wherein the electric field of saidRF wave is decreasing, whereby said optical pulse is downshifted to alower optical frequency.
 8. The method of shifting the wavelength ofoptical radiation of a substantially discrete wavelength, comprising thesteps of(a) propogating said optical radiation through anelectro-optical medium, (b) generating an RF traveling wave having adesired phase relationship with said optical radiation, (c) propogatingsaid RF traveling wave in association with said electro-optical medium,whereby the instantaneous electric field of said RF traveling waveinteracts with said electro-optical medium locally therein to shift theoptical index of refraction of said electro-optical medium as a functionof space and time.
 9. The method of claim 8 further comprising the stepsof periodically re-establishing said desired phase relationship.
 10. Themethod of claim 9 wherein said step of periodically re-establishingcomprises inverting the instantaneous phase of said optical radiation insaid electro-optical medium in respect of the phase of said RF travelingwave.
 11. The method of claim 9 wherein said step of periodicallyre-establishing comprises inverting the instantaneous phase of said RFtraveling wave in respect of the phase of said optical radiation. 12.Apparatus for spectral analysis of selected atomic constituents of asample comprising(a) a light source for emitting light having a spectralcharacteristic of said selected contituent, (b) atomizing means forforming and directing a cloud of an atomized form of said sample intothe path of a beam of light emitted by said light source, (c) opticalwavelength shifting apparatus comprising:an optical guide meanscomprising an electro-optic medium, a traveling wave guide structure forpropogating an RF wave, said optical guide disposed in relation to saidtraveling wave guide structure whereby the refractive index of saidelectro-optic medium is locally responsive in proportion to thecorresponding local electric field set up by the traveling wave to shiftthe wavelength of said traveling wave, said optical pulse synchronizedin phase with said RF traveling wave whereby the leading edge of saidpulse and the trailing edge of said pulse is substantially within thesame RF (d) photoelectric detection means for detecting and measuringthe intensity of a selected spectral emission line of the emissionspectrum of said light source after said line has transited saidwavelength shifter and said atomized sample, (e) modulation meansoperative upon said wavelength shifter to vary said refractive index ina selected manner.